In recent years, polyester films have been frequently used as various kinds of optical films, e.g., employed in various applications such as an antireflection film, a touch panel, a prism sheet, a light diffusion sheet and an electromagnetic shielding film as a member of liquid crystal displays or plasma displays. A base film used for these members has been required to have excellent transparency and visibility.
These films have been often subjected to hard coating treatment in order to enhance an anti-curling property and a mar resistance thereof, etc. In the hard coating treatment, in order to enhance adhesion between a polyester film as a base material and a hard coat layer, a coating layer having an easy-adhesive property is generally provided as an intermediate layer therebetween. For this reason, refractive indices of these three layers including the polyester film, the easy-adhesive coating layer and the hard coat layer must be taken into consideration to avoid occurrence of interference fringes and deterioration in visibility.
In recent years, form the economical viewpoints, it has been required to take a suitable measure against the interference fringes by providing a high-refractive hard coat layer to simplify a structure of an antireflection layer to be formed thereon, or by providing the high-refractive hard coat layer solely without any antireflection layer. For this reason, the easy-adhesive coating layer is also required to have a high refractive index. In general, it is considered that the refractive index of the coating layer at which occurrence of interference fringes can be reduced is a geometrical mean value of a refractive index of the polyester film as a base material and a refractive index of the hard coat layer. Therefore, it is required to adjust the refractive index of the coating layer near to the geometrical mean value.
As the method of enhancing a refractive index of the coating layer to prevent occurrence of interference fringes, there is known, for example, the method in which a metal oxide having a high refractive index is incorporated in the coating layer. However, in this method, the resulting film tends to be deteriorated in transparency, thereby failing to exhibit a sufficient performance thereof in various optical applications (Patent Document 1). As an alternative method, there has been proposed the method in which a metal chelate compound is used in combination with a resin. However, in this method, owing to unstableness of the metal chelate compound in an aqueous solution, a coating solution used therein also tends to become unstable according to combination between the compound and resin, so that the procedure of replacing the coating solution with new one must be conducted so many times while continuing the film production process for a long period of time (Patent Document 2). In addition, an ordinary high-refractive material tends to be deteriorated in adhesion to a surface functional layer such as the hard coat layer. Therefore, it has been required to provide a coating layer which is capable of effectively exhibiting an enhanced adhesion property to the high refractive material even when used in combination therewith.